In multipath transmission systems, such as those which utilize troposcatter or high frequency (e.g. 3-30 MHz) communication links, for example, the transmitted signal is conveyed through a medium along a plurality of paths of differing lengths so that a plurality of signals, each representing the transmitted signal but having varying energy contents, are received at the receiver at different times depending on the length of each particular transmission path in the medium. In some applications redundant, or diversity, channels each containing a plurality of differing path lengths are used to decrease the vulnerability to fading effects.
A general discussion of such communication systems and the problems associated therewith can be found in a recently issued paper "Fading Channel Communications" by Peter Monsen, published in IEEE Communications Magazine, January 1980, pages 16-25. Such paper discusses general approaches to receivers for use in such circumstances and particularly discusses various prior art approaches to adaptive receiver structures, particularly for preventing intersymbol interference.
A more specific discussion of a particular communications receiver for processing received diversity signals and eliminating intersymbol interference is described in U.S. Pat. No. 3,879,664, issued Apr. 22, 1975 to Peter Monsen. In such system the receiver utilizes a forward adaptive filter equalizer, having a plurality of weighting sections, in each of the receiver diversity channels for processing each of the received diversity signals. The combined weighted output signal is used to reconstruct the transmitted data and to generate an error signal. The error signal is used to derive appropriate adaptive weighting signals for use in the processing of the received diversity signals at each of the forward filter equalizers. The forward filter equalizers provide for appropriate matched filtering of the received signals and tend to reduce future intersymbol interference. The system further utilizes a single backward filter equalizer which suitably processes the reconstructed data to form a cancellation signal which is effectively used to eliminate past intersymbol interference. The same error signal is also used to derive appropriate weighting signals in the backward filter equalizer.
While such a system as described in the aforementioned issued U.S. patent has been found extremely useful in troposcatter applications, in some applications the adaptive filter techniques do not provide a sufficiently fast convergence process so that errors due to intersymbol interference are not reduced as quickly as is desirable in some cases. For example, the particular system described therein becomes less helpful in high frequency digital data communication systems because the ratio of data rate to channel rate of change is not so large as to provide sufficient learning time for proper adaptation. Relatively slow convergence of the adaptive techniques tends to occur because the weighting signals within the forward and backward filter equalizer circuits are correlated so that the error signal that causes a change in one weight will tend to change the weights in the other components of the filter.
Accordingly, for high frequency data communication receivers it is desirable to provide an adaptive filter technique design which promotes faster convergence by reducing or eliminating the correlation between the weighting signal components.